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Soil Temperature
Soil temperature influences seedling growth and survival by influencing physiological processes such as respiration and water uptake by roots. Constraints on plant growth arise from temperature extremes. (Fluctuations in soil temperature tend to be greatest at or near the soil surface, which can be of crucial importance.)
Low soil temperature is a major is a major constraint on rooting depth and root growth. It is also probably the most important environmental factor that influences the rate of water uptake. Newly planted seedlings initially have poor contact between their roots and the soil. This can be further aggravated by low soil temperature, which increases the viscosity of water and thus it's resistance to flow. Higher soil temperature stimulates rapid root development and thus promotes good root-soil contact, which can help alleviate seedling water stress. The optimal soil temperature ranges for root growth of "Boreal" spruce and pine species are approximately 20-25C and 25-30C, respectively.
Low soil temperature at the soil surface increases the risk of frost damage to aerial tissues and is associated with frost heaving, a major cause of seedling mortality. Air temperatures of approximately -3C during the growing season will result in frost damage to foliage or succulent stem tissue.
Solar radiation has the greatest influence of all factors on soil temperature. Solar radiation is the amount of solar energy absorbed by the surface material ( a dark surface absorbs more energy) and the amount of light intercepted by a surface layer such as vegetation or a litter (mulch) (e.g., organic material or dry, coarse sand).
High soil surface temperatures under "Boreal" conditions are most common near seedlings and young plants, whose succulent stems are in contact with an exposed and highly insulated surface soil of low heat conductivity that can warm, such as dry coarse sand or duff. Seedlings are at risk of damage from high temperatures primarily in a zone about 1 cm (1/2 inch) above and below ground level.
Soil temperatures are determined by weather (Macroclimate), in the form of solar radiation and precipitation (soil moisture content), and site characteristics; site affects soil temperature through it's location (slope and aspect), ground cover (vegetation and duff), and the physical properties of the soil (soil texture and moisture).
Heat storage, heat transfer, and temperature within the soil profile are determined by the thermal conductivity and the volumetric heat capacity of the soil (both increase as the soil water content increases). Consequently, wetter soils (fine textures soils are often wetter than coarse soils) warm more slowly and are generally colder than drier soils in the same area. Coarse-textured soils tend to create a surface mulch more readily than do fine-textured soils. Temperature changes are transmitted slowly through very dry or very wet soils.
